1. A ‘field’ is a region in space where a force may act.
Currently you are in a Gravitational Force field….

2. Gravitational Fields Gravity = Field Force Gravity = Non-contact force
A gravitational field is region in space where gravitational forces can be detected.
It can be depicted using gravitational field lines.
The direction of the field is shown by an arrow and is the direction a mass would move if placed in the field.
The density of the field lines shows the strength of the gravitational field.

3. Web Links: Releasing a test charge
An electric field is defined as being present in any region where a charged object experiences an electric force. 
Web Links: Releasing a test charge

4. An electric field describes a region where an electric charge will experience a force if placed in that region.
The electric field can be represented by field lines. These lines start on a positive charge and end on a negative charge.
The direction of the field lines show the direction of the electrostatic force that would act on a positive charge, q,
if you placed it there.

5. Electric field lines never cross and are drawn perpendicular to a surface.
The electric field is stronger where the field lines are closer together.

7. 4) E-fields add as vectors
Notes on E-field
1) The E-field points in the direction of force on a positive test charge
2) If a negative charge were placed in the E-field, what do you suppose would happen?
3) The E-field is a property of the fixed charges only (it is independent of the test charge)
4) E-fields add as vectors
5) Given the E-field value at a certain point, we can calculate the force F on any charge q0 placed there:
F = q0E
© Laura Fellman

9. Uniform Electric Field+ -
Evenly spaced field lines
Parallel field
lines
In a uniform field, the electric field lines are drawn parallel and evenly spaced.

10. See Charges And Fields PHET applet

11. The Electric Field strength (or intensity) is defined as the force on a test charge (small positive charge), divided by the charge: E E F F
Electric field of a point charge +Q r

13. Electric Field Strength+ - +Q F
Symbol E
Defined as the force acting per unit of positive charge
Measured in N/C or V/m
Vector - directed from positive to negative

14. 1) A proton is released in a uniform electric field, and it experiences an electric force of 3.75 × N toward the south. What are the magnitude and direction of the electric field? E +
south F

15. F = qE = 1.6x10-19 (6x104) = 9.6x10-15 N, upward FG = mg
2) The electric field intensity between two plates is constant and directed downward. The magnitude of the electric field intensity is 6x104 N/C. What are the magnitude and direction of the electric force exerted on an electron projected horizontally between the two plates?
F = qE
= 1.6x10-19 (6x104)
= 9.6x10-15 N, upward
E = 6x104 N/C
qe = 1.6x10-19 C
Show that the gravitational force on the electron above may be neglected.
FG = mg
= 9.11x10-31 (9.8)
= 8.92x10-30 N
me = 9.11x10-31 kg
The electric force is larger than the gravitational force by a factor of 1.08x1015

16. The electric field intensity E at a distance r from a single charge q can be found as follows:
Units: N/C
3) What is the electric field intensity at a distance of 2 m from a charge of -12 μC?
r = 2 m
q = -12 μC
= 27x103 N/C, towards q

17. 4) What are the magnitude and direction of the electric field at a point midway between a +7.0 mC and a -8.0 mC charge 8.0 cm apart? Assume no other charges are nearby. + - d q1 q2 E1 E2

18. 5) Two point charges q1 = -6 nC and q2 = +6 nC, are 12 cm apart, as shown in the figure. Determine the electric field b. At point B E2
q1 = -6x10-9 C
q2 = +6x10-9 C
37º θ ER E1
= 6.67x103 N/C
= 2.4x103 N/C

19. From vector diagram: Σ Ex = - E2cos 37˚ = - (2.4x103)(cos 37˚)
37º
Σ Ex = - E2cos 37˚
= - (2.4x103)(cos 37˚)
= N/C θ ER E1
Σ Ey = E2 sin 37˚- E1
= (2.4x103)(sin 37˚) - (6.67x103)
= N/C
= N/C

20. = 70˚ E2
37º θ
180˚ + 70˚ = 250˚
ER (5566 N/C, 250˚) ER E1